1. Field of the Invention
The present invention relates to a method and apparatus for substantially reducing field strength non-uniformities in the bore of the high field magnet and is particularly useful for homogenizing the primary magnetic field of a magnetic resonance investigatory device, such as a magnetic resonance imager or spectrometer.
2. Description of the Prior Art
Investigative apparatus, such as a magnetic resonance imager (MRI) device or spectrometer, determine characteristics of a test object by subjecting the object to a high (primary) magnetic field and RF-pulses and then examining the resolvable MR spectra. Examination is carried out by detecting precessions of the various classes of protons of the test object caused by the magnetic fields and RF-pulses. In order for the detected precessions to accurately reflect resolvable characteristics of the full volume of the test object, all portions of the test object must be subjected to a primary magnetic field of uniform field strength. This is particularly true for physically large MRI devices, such as those designed for imaging various portions of a human being, and even for small magnetic resonance devices which have a high field strength designed for determining the chemical makeup of the test object, i.e., a field strength in the order of 1-10 tesla, since even slight magnetic field non-uniformities at these field strengths leads to inaccurate results after processing the detected resonance signals.
U.S. Pat. No. 3,564,398 issued Feb. 16, 1971 to Nelson discloses one type of prior art technique for homogenizing a magnetic field, comprising a plurality of correction coils which are independently positioned within the primary field and energized so as to cancel certain field strength non-uniformities of the field. This technique suffers from the disadvantage that the correction coils necessarily increase the mechanical and electrical complexity of the magnetic resonance device and furthermore require a separately controllable and highly regulated correction coil power supply. Additionally, the reliability of the magnetic resonance device is reduced and its accuracy is in jeopardy if the correction coil power supply should have a fluctuation or failure.
U.S. patent application Ser. No. 742,811 of Frese and assigned to the same assignee as the present invention, uses strategically placed ferromagnetic sheets (i.e., iron plates) in close proximity to the magnet winding in order to reduce magnetic field non-uniformity. This technique, although being less complex and therefore inherently more reliable than the above-noted correction coil technique, is inconvenient in situations where it may be necessary to change the position and/or number of the iron plates in order to correct for a changed magnetic field strength of the device or a physical repositioning of the device after its initial installation. Furthermore, in small bore magnets, it may be impossible to gain access to the iron plates located on the inside of the bore, and in large bore magnets, the device must be shut down in order to allow entry of workmen to reposition and secure the iron plates in new locations. The down-time associated with the repositioning of the plates takes the device out of service for a substantial period of time and is therefore relatively inconvenient and costly to the owner.
It is an object of the present invention to provide a method and apparatus for substantially reducing magnetic field strength non-uniformities which is mechanically simple, does not consume power, has long-term reliability and which is relatively easy to adjust in the event of changes in the magnetic field non-uniformities to be corrected.